A complete topological equivalence between network representations of mechanical and electrical systems is hindered by the inertial mass element, represented as an equivalent capacitor with a pin always connected to ground (inertial reference system). This limits the class of synthesizable mechanical filters from equivalent electrical prototypes by excluding the schematics that have floating capacitors. This restriction motivates the introduction of a differential mass component. It is shown that this new component would make it possible to implement a simple third order Chebyshev bandpass filter which can not be implemented using standard masses, springs and dampers.